The present invention relates to a binaphthol bistriflate, 2,2xe2x80x2-bis(trifluoromethanesulfonyloxy)-1,1xe2x80x2-binaphthyl. This compound is useful as a precursor of 2,2xe2x80x2-bis(diphenylphosphino)-1,1xe2x80x2-binaphthyl (BINAP), which is an important chiral ligand for catalytic asymmetric synthesis.
Each of Tetrahedron Letters, Vol. 31, No. 7, pp. 985-988, 1990, U.S. Pat. No. 5,399,771 and Organic Syntheses, Vol. 76, pp. 6-11 discloses a process for producing binaphthol bistriflate by reacting binaphthol with trifluoromethanesulfonic anhydride in the presence of an organic base.
WO 99/36397 discloses a process for producing binaphthol bis(perfluoroalkanesulfonate) derivatives, which are precursors of BINAP derivatives. In this process, binaphthol derivatives are reacted with a perfluoroalkanesulfonyl halide, CnF2n+1SO2X, where n is an integer of 4-10 and X=F or Cl, or a perfluoroalkanesulfonic anhydride, (CnF2n+1SO2)2O, where n is an integer of 4-10, in the presence of an organic base.
As mentioned above, trifluoromethanesulfonic anhydride is used as a trifluoromethanesulfonylation agent for producing a binaphthol bistriflate. This anhydride is produced by obtaining trifluoromethanesulfonyl fluoride through electrolytic fluorination and then by hydrolyzing the trifluoromethanesulfonyl fluoride into trifluoromethanesulfonic acid, followed by dehydrocondensation, as follows.
CF3SO2Fxe2x86x92CF3SO3Hxe2x86x92(CF3SO2)2O
The molecule of trifluoromethanesulfonic anhydride contains two trifluoromethanesulfonyl groups. However, only one of these groups is introduced into binaphthol, since the other group acts as the leaving group (TfOxe2x88x92=CF3SO3xe2x88x92). Therefore, trifluoromethanesulfonyl fluoride is superior to trifluoromethanesulfonic anhydride as a trifluoromethanesulfonylation agent. It is, however, very difficult to handle trifluoromethanesulfonyl fluoride, since this compound (boiling point: xe2x88x9220xc2x0 C.) is highly volatile at normal temperature and normal pressure. In connection with this, WO 99/136397 discloses a perfluoro-1-butanesulfonylation at normal temperature and normal pressure using a low volatile perfluoroalkanesulfonylation agent, such as perfluoro-1-butanesulfonyl fluoride (boiling point: 64xc2x0 C.), without using a special reaction apparatus such as pressure-proof reaction vessel. However, the introduction of a leaving group having many fluorine atoms is disadvantageous from the viewpoint of atom economy, since the final object is to synthesize BINAP derivatives.
It is an object of the present invention to provide a process for producing a binaphthol bistriflate, which does not require a special reaction apparatus such as a pressure-proof reaction vessel, although the process employs trifluoromethanesulfonyl fluoride, which is highly volatile at normal temperature (e.g., room temperature) and normal pressure (e.g., atmospheric pressure).
According to the present invention, there is provided a process for producing a binaphthol bistriflate represented by the formula (1). This process comprises reacting a binaphthol represented by the formula (2) with trifluoromethanesulfonyl fluoride, in a polar solvent, in the presence of an organic base. 
The inventors unexpectedly found that the reaction proceeds quickly at a low temperature by using a polar solvent as the reaction solvent, while the reaction vessel""s pressure does almost not increase. Therefore, the process of the invention does not require a special apparatus such as a pressure-proof reaction vessel.
Binaphthol is a starting material in the process of the invention. This compound may be R-form, S-form or racemate in configuration. The racemization does not occur in the reaction. Therefore, it is possible to obtain a binaphthol bistriflate of R-form, S-form or racemate by suitably selecting stereoisomer of the binaphthol.
Trifluoromethanesulfonyl fluoride is used in the reaction as a trifluoromethanesulfonylation agent. This fluoride is in an amount preferably of at least 2 moles, more preferably 2-10 moles, still more preferably 2-5 moles, per mol of the binaphthol.
Nonlimitative examples of the organic base used in the reaction are trimethylamine, triethylamine, diisopropylethylamine, tri-n-propylamine, tri-n-butylamine, dimethyllaurylamine, dimethylaminopyridine, N,N-dimethylaniline, dimethylbenzylamine, 1,8-diazabicyclo(5,4,0)undecene-7, 1,4-diazabicyclo(2,2,2)octane, pyridine, 2,4,6-trimethylpyridine, pyrimidine, pyridazine, 3,5-lutidine, 2,6-lutidine, 2,4-lutidine, 2,5-lutidine, and 3,4-lutidine. Of these, trimethylamine, triethylamine, diisopropylethylamine, and tri-n-propylamine are preferable. In particular, triethylamine is more preferable. The amount of the organic base used in the reaction is preferably at least 2 moles, more preferably 2-10 moles, still more preferably 2-5 moles, per mol of the binaphthol.
Nonlimitative examples of the polar solvent used in the reaction are N,N-dimethylformamide (DMF), N,N-diethylformamide, N,N-dimethylacetoamide, 1-methyl-2-pyrrolidinone, 1,3-dimethyl-2-imidazolidinone, hexamethylphosphoric triamide, dimethyl sulfoxide, acetonitrile (CH3CN), propionitrile, and benzonitrile. Of these, N,N-dimethylformamide, N,N-dimethylacetoamide, 1-methyl-2-pyrrolidinone, and acetonitrile are preferable. In particular, N,N-dimethylformamide and acetonitrile are more preferable. It is optional to use one of these compounds or a mixture of at least two of these in the reaction.
As stated above, the gist of the invention is to use polar solvent as the reaction solvent. An advantageous effect of the use of polar solvent is discussed in detail as follows. In case of using a highly volatile reagent in a reaction, it is generally important to quickly proceed the reaction at as low a reaction temperature as possible, With this, it becomes possible to minimize load to the reaction vessel in pressure. In other words, it becomes unnecessary to use a special reaction vessel such as pressure-proof reaction vessel. The results of the after-mentioned Examples 1-4 and Comparative Examples 1-2 are summarized in Table with respect to conversion of the reaction.
As shown in Table, methylene chloride (CH2Cl2) was used as a comparative solvent in Comparative Examples 1 and 2; methylene chloride is used as the most preferable reaction solvent in WO 99136397. It is understood from Table that the reaction proceeds quickly at low temperature by using polar solvent. The temperature of the reaction mixture was maintained at xe2x88x9230xc2x0 C. for 1 hr in Examples 1 and 2. In contrast, it was increased from xe2x88x9230xc2x0 C. to 0xc2x0 C. and maintained at 0xc2x0 C. for 1 hr in Examples 3 and 4. Even in Examples 3 and 4, the reaction vessel""s pressure did almost not increase during the reaction, since most of the trifluoromethanesulfonyl fluoride was already consumed in the course of the temperature increase toward 0xc2x0 C. Therefore, it was not necessary to use a special apparatus, such as a pressure-proof reaction vessel, is even in Examples 3 and 4. The reason why the reaction proceeded quickly at low temperature can be that the solubility of binaphthol (particularly its racemate which is less soluble in solvent) at low temperature improved remarkably by using polar solvent and that the rate of the trifluoromethanesulfonylation itself was accelerated by using polar solvent.
It suffices that the amount of polar solvent used in the reaction is in an amount sufficient for dissolving therein binaphthol in an amount sufficient for quickly proceeding the reaction at low temperature. Its amount is preferably from b 1 to 10 liters, more preferably from 1.5 to 5 liters, per mol of binaphthol.
The reaction temperature is not particularly limited so long as it is in a range of from a first temperature, at which trifluoromethanesulfonyl fluoride (boiling point: xe2x88x9220xc2x0 C.) can be introduced in liquid state, to a second temperature, at which the reaction vessel""s pressure does not become too high. The reaction temperature is preferably from xe2x88x9260 to 10xc2x0 C., more preferably from xe2x88x9250 to 0xc2x0 C.
It is possible to obtain a crude product of the binaphthol bistriflate by conducting a conventional post-treatment after the reaction. According to need, it is possible to obtain the binaphthol bistriflate of high purity by conducting a purification such as purification with activated carbon, recrystallization or column chromatography.
The following nonlimitative examples are illustrative of the present invention.